700 THERMOMETER structed, and at the expiration of that period the readings may all be too low by nearly a degree ; hence it is necessary either to add the proper correction to the readings of the thermometer, or to slide down and refix the scale to which the thermometer is attached, so that it will read accurately. Alcohol, com- monly used where temperatures much below F. are to be observed, is liable at such range to much variation, although it does not freeze even at 132 F.; and Capt. Parry, in his arctic voyages, observed differences of full 10 C. between alcohol thermometers by the best makers. Self-recording Thermometers. Vari- ous instruments have been invented which record the indications of the thermometer. They may be divided into two classes, those which record only the maximum and the mini- mum of the temperatures occurring in any definite period, and those which produce con- tinuous records. In the first class may be mentioned the two following instruments. An ordinary mercurial thermometer has its tube constricted to a thin passage at some point between its bulb and the beginning of its 8cale. This thermometer is placed in a hori- zontal position, and then as long as an increase of temperature takes place small portions of FIG. 2. Maximum and Minimum Thermometers. the mercury will go in a series of jumps across the constricted passage; but on a fall of temperature the mercury contracts into the portion of the thermometer below the con- striction, leaving a column of mercury above it. The upper end of the latter column marks the highest temperature reached during the time of exposure. To readjust this instru- ment, the mercury is sent into the vacant space below the constriction by swinging the instru- ment. Fig. 2 shows this "maximum ther- mometer," the invention of which has been claimed by several persons. The "minimum thermometer " of Rutherford, which is gener- ally used, is made of alcohol contained in the ordinary glass bulb and tube! In the column of alcohol is a small index made of black glass and shown at 71, fig. 3. This piece of glass is brought up to the end of the fluid column by inclining the instrument. The thermome- ter is then placed in a hori- zontal position, and as the temperature falls the top of the liquid column during its retraction carries the glass index with it, and leaves it at the point which indicates the min- imum temperature reached during the expo- sure of the instrument. The thermometers of Fio. 8. Index of Butherford's Ther- mometer. THERMOPYLAE the second class give continuous records, either by causing a tracer attached to some simple or compound metallic bar to mark a continuous line on a cylinder which revolves once in 24 hours, or by the aid of photography a contin- uous impression of the image of the top of a thermometric column is obtained by illumina- ting a thermometer placed in front of the lens of a camera, while at the back of the camera is a sensitized plate on which the image is formed. The plate traverses athwart the beam issuing from the lens by a known distance each hour. Differential Thermometer. This is a modification of the air thermometer, in which two large glass bulbs above are connected by a glass tube bent twice at right angles ; the hori- zontal and parts of the upright tubes are filled in the common form with a colored liquid, which is depressed on either side as the corre- sponding bulb is more heated ; thus the instru- ment indicates differences of the temperatures to which the two bulbs may be exposed. It is very sensitive ; and by a scale the results it affords are comparable with each other. THERMOPYLAE, or simply Pytee (from 0ep//<fc, hot, and My, gate), a defile between Thessaly and Locris, in antiquity the only passage for an enemy from northern into central Greece, situated between Mt. (Eta and an inaccessible morass forming the edge of the Maliac gulf, and containing several hot springs. There was a road wide enough only for a single wheel track, which formed the western gate. About a mile to the eastward Mt. (Eta again approached the sea in a similar manner, and the passage there formed the eastern gate. The space between these two gates was wider, and many years before Leonidas occupied the pass, the Phocians had so conducted the warm springs over the ground as to render the pass impracticable. They had also built a wall near the western gate to prevent the incursions of the Thessalians, which was in ruins when the Spartans came. This pass is celebrated for its defence against the army of Xerxes by the Greeks under the Spartan king Leonidas, in 480 B. 0. His forces numbered probably about 7,000 ; but when during the battle he learned that one Ephialtes, a Thessalian, had betrayed to the Persians a circuitous path over the mountains leading to their rear, he dismissed all but his chosen band of 300 Spartans, with a number of helots, about 700 Thespians who volunteered to share his fate, and apparently 400 Thebans. This small host sallied out and fought till Leonidas and all the Spartans and Thespians were killed. The fate of the The- bans is uncertain ; according to some they sur- rendered to the Persians. One Spartan, Aris- todemus, who was prevented by illness from partaking in the combat, returned home, and was received with scorn, but in the following year retrieved his honor by a heroic death at
